It is already known that certain kinds of glass are obtained from mixtures of mineral oxides which also included heavy metals oxides, such as lead oxides. In particular, crystal, by definition, contains at least 24% lead oxide. In this description, the term "crystal" will designate lead glass of whatever lead content, and in a general way any glass containing heavy metals.
When glass is in contact with products, particularly food products, a migration of some mineral elements of the glass is observed, which, although vitreous in nature, is not totally chemically inert. This process has been observed in glass receptacles containing liquids such as water, and especially liquids having an acidic pH. The case of crystal receptacles has been the object of high-level research due to the migration of lead and to the so-called risks to which it exposes consumers. Regulations are appearing in a number of countries to limit the maximum lead content of drinks after coming into contact with crystal receptacles. For example, in California, the value over which a specific indication becomes obligatory has been fixed (proposition 65) at 25 ppb (parts per billion) measured according to Standard ISO 7086. It should also be noted that the standard currently in force in the USA sets the maximum lead content in alcoholic drinks at 300 ppb.
The method for measuring the migration of lead is standardized by Standard ISO 7086, and consists in measuring the lead content in a 4% acetic acid solution after being left to stand for 24 hours at an ambient temperature of 22.degree. C.
A great deal of effort is now being channeled into finding processes for treating crystal surfaces in order to block the migration of heavy metals, especially lead, towards the products with which the crystal is, in contact. This concerns a large number of articles of the goblet and glass type, and all kinds of receptacles such as flasks, carafes, jugs, etc.
Surface treatments are also being sort that will better conserve the surface state of articles that are subjected to frequent washing in automatic dish washing machines.
An example of bibliographic references illustrating the state of the art in the field of the invention, especially the migration of lead into products in contact with crystal, is represented by the Ceramic Bulletin volume 55 No. 5 (1976) pages 508 and following, which shows the influence of the liquid's pH on the migration of lead. An article in the Journal of the Am Ceramic Society Volume 61 No. 7,8 (1978) pages 292 and following is devoted to a theoretical study of the determination of the activation energy of the ionic exchange between the H.sup.+ ions of the liquid and the Pb.sup.++ ions.
Several techniques have already been defined to limit lead migration into products, particularly with respect to liquids brought into contact with crystal articles,
An initial suggestion was to modify the composition of the glass itself. For example, the suggestion was made to modify the Al.sub.2 O.sub.3 content of a crystal: refer, for example, to SCHAEFFER and al in Berichte der deutschen Keramischen Gesellschaft Volume 53 (1976) No. 2 pages 43-78, who observed that the introduction of 3% Al.sub.2 O.sub.3 into a crystal was sufficient to reduce the speed and quantity of migrating lead by a factor of 10. This type of technique, however, is not easy to carry into practice since it involves complete modification of the glass compositions which results in high costs because of the disruption caused to the manufacturing process.
Another technique consists in conserving the conventional composition of the glass or crystal, but performing a surface treatment in order to block the migration of lead. By way of example, in patent application EP-0 458 713, the surface to be treated is coated with a suspension of kaolin to provide a surface film, which is then treated at high temperature, the film then being removed, once the article has cooled, by washing in water and then by sand or ultrasound. This document also contains a detailed description of the technical problems that are encountered in the field of the invention. The drawbacks of the process described in this patent application EP-0 458 713 reside in its carrying into practice, which calls for a barbotine to be casted on the surface to be treated, and above all in the operations for removing the film which are both long and costly and which may also degrade the surface state of the article.
Document DE-A-41 02 886 describes the treatment of glass articles in order to limit the transfer of the heavy metals that they contain. The procedure involves the use of an extraction agent in an aqueous solution and is performed at low temperature, notably less than 100.degree. C.
Other types of treatment exist such as treatments using concentrated sulfuric acid, but the results obtained are unsatisfactory and unstable in time.
The surface treatment technique using ammonium sulfate has been known for a long time, and has been the object of several recent papers, particularly at the 1992 International Crystal Federation (ICF) congress (Technical Exchange Conference). According to the authors of these papers, treatments using ammonium sulfate give good results for slowing down lead migration.
The Applicant set out to find an improved process coming within the scope of the general technique of the surface treatment of glass articles containing heavy metals, that is easy to carry into practice and which gives results hitherto unattained in blocking the migration of lead into the products, especially liquids, brought into contact with said articles.